1. Field of the Invention
The present invention pertains to self-propelled hovercraft. More specifically, the present invention pertains to apparatus for inhibiting plowing in of hovercraft and increasing the stability thereof when such hovercraft encounters sudden increased surface contact involving the peripheral skirt. In particular, the present invention pertains to improved segmented skirt apparatus designed to increase stability of the hovercraft and to inhibit plowing in of the hovercraft in response to sudden increased surface contact, particularly in water.
2. Description of the Prior Art
"Hovercraft" refers to an amphibious version of a more general air cushion class of vehicles, sometimes called ground effect machines. Hovercrafts are machines which slide along the surface of the ground or a body of water while balancing on top of an air cushioned bubble. The air bubble is generated by a power driven fan, a portion of the air from which is directed into an air plenum provided on the underneath side of the hovercraft body. A flexible skirt of some type is provided around the machine to retain the bubble beneath the machine by limiting peripheral air loss. Lubrication provided by the air bubble enables the machine to slide across relatively flat and smooth surfaces such as grass, snow, ice, water, mud, sand, and even some crops growing therefrom. The power driven fan supplies not only enough air to lift the machine on its cushion or bubble of air but also the power necessary for propelling the vehicle across the ground or water above which it is supported.
The skirts of early hovercraft were of several designs. Some skirts consisted of brush stubble attached around the edge of the machine. On some machines the brush height could be adjusted. Brush skirts proved to be unstable, stiff and ineffective and are no longer in use.
In 1959, Jene Bertin, a French engineer--inventor, invented the jupe skirt, sometimes referred to as the "cell skirt". The jupe or cell appears as the frustrum of a cone resting upside down. It slopes toward the bottom and inflates in a conical shape. Although the jupe or cell skirt is not widely used, some hovercraft use this design.
Until recent years, the most widely used hovercraft skirt was the bag skirt which took the shape of a giant inner tube fitted around a craft's perimeter. Air pressure is needed to inflate the bag and maintain inflation against the air pressure of the air cushion under the craft.
In more recent years, the convoluted, segmented, or finger skirt has found widespread use in Europe and the United States. Almost all European and American hovercraft are fitted with such segmented skirts. With the segmented skirt, a plurality of bags, one adjacent to another, completely encircles the hovercraft body. Each of the bags, having front and side portions, is in fluid communication with an air plenum and, together, provide a flexible seal between the air cushion under the hovercraft and the ground or water across which the hovercraft is to be propelled. The individual bags, made of flexible material, allow the hovercraft to slide across the terrain, even though the terrain is not perfectly smooth and may have objects, such as stumps, projecting upwardly therefrom, allowing the collapse or displacement of one or two bags without disturbing others. The segmented skirt is thus a substantial improvement over the prior art.
Although the segmented skirt hovercraft is the most widely accepted design, it still has problems, particularly when traveling across water. When a hovercraft encounters sudden increased surface contact, particularly in water, it has a tendency to "plow" into the water. Plowing in is a very undesirable phenomena in which the front hull of the hovercraft makes sufficient contact with the water surface to cause rapid deceleration and instability (loss of control). The plowing in phenomena appears to be due to a combination of effects from variations in skirt drag and forward weight transfer. This transition from normal operation to plowing in occurs when, for any reason, a sudden increase in water surface drag on the skirts causes craft deceleration and transfer of weight to the front of the craft. This pushes the front skirts into greater surface contact with the water, resulting in even greater drag and deceleration. There is a chain reaction which proceeds quickly to the point where the hull of the hovercraft if forced into the water surface causing very rapid deceleration and loss of control.
In an effort to overcome the plowing in phenomena, prior art designs have attempted to increase air flow and pressure at the front of the hovercraft and by providing means of transferring weight in the hovercraft or with aerodynamic controls to maintain a "nose up" operational attitude. While these approaches have been somewhat successful, further improvements are needed.